This invention relates generally to a method and apparatus for separating particles fluidly suspended in a slurry and, in particular, to separating particles suspended in slurry extracted from oil wells.
During an oil drilling operation, drilling "mud" is injected into the oil well to maintain the pressure therein and for other reasons recognized by persons of ordinary skill in the art. The drilling mud, hereinafter called slurry, is extracted from the oil well on a regular and/or continuous basis. The extracted slurry contains a number of different "contaminants" such as quartz sand and other formation solids suspended in the slurry. The slurry, before injection, includes barite (BaSO.sub.4), bentonite or other special clays known to persons of ordinary skill in the art. The desirable components of the drilling mud, i.e., the barite and/or bentonite, can be recycled and utilized in refurbished slurry to be injected into the oil well if the formation solids and particularly the quartz sand is removed from the contaminated drilling mud or slurry.
A number of different methods and apparatus have been developed to separate out the quartz sand and bentonite from the slurry. One known device is a cyclone or centrifugal particle separator which is sometimes called a "hydrocyclone" if water is part of the slurry. The cyclone separator usually includes a cylindrical region into which the contaminated fluid under pressure is tangentially injected, and a frustoconical region adjacent the cylindrical region. The standard cyclone separator includes a sealing member or plate at the one opposite axial end of the cylindrical region. The base of the frustoconical region adjoins the other, opposite axial end of the cylindrical region. A typical hydrocyclone includes an underflow or lower outlet port at the truncated apex of the frustoconical region. Also, the standard cyclone separator includes a cylindrically shaped vortex finder defining an upper outlet port which is in communication with the cylindrical region and which extends through the sealing member at that axial end. Both the lower and upper outlet ports are concentric to the longitudinal axis of the cyclone separator. As utilized herein, the axis of the cylindrical region and frustoconical region is the axial centerline of those geometrically defined shapes. The term "axially inboard" (and "axially outboard") refers to components or items axially positioned closer to (or further away from) the plane normal to the axis of the cylindrical and frustoconical regions which intersects the axial midpoint between the axial extent of the combined cylindrical and frustoconical regions.
The injected slurry, carrying the fluidly suspended particles, enters the cylindrical region under a continuous pressure head. The slurry in the cylindrical region swirls about in a spiral-like fashion and enters the frustoconical region. This spiral-like motion at the radial outer sectors of the frustoconical region is recognized as the "outer vortex" or the "free vortex" by persons of ordinary skill in the art. The velocity of the particles carried by the slurry in the free vortex is continuously increased due to the increasingly smaller radial dimension within the frustoconical region. A substantially continuous flow of slurry will exit the lower outlet port. However, because of the centrifugal forces developed within both the regions, a flow of slurry develops at the radially inward sectors of the frustoconical region in a helical or spiral-like path directed towards the cylindrical region. This type of flow is called, by persons of ordinary skill in the art, the "inner vortex" or "forced vortex." Also, an air core coaxially develops along the axis in both regions.
Two types of forces act upon the particles entrained or carried by the slurry flowing within the cyclone separator, to wit, the drag forces of the liquid acting on the individual particles in the slurry and the centrifugal settling forces which effect the radial positioning of the particles. Therefore, particles having relatively higher drag forces and comparatively lower centrifugal settling forces generally move towards the radially inner sectors of the cyclone due to its defined internal geometric shape and become entrained in the forced vortex and hence are extracted from the cyclone with the forced vortex portion of the slurry via the vortex finder defining the upper outlet port. On the other hand, particles having relatively lower drag forces and comparatively higher centrifugal settling forces generally move to the radially outermost sectors of the cyclone separator and are entrained in the free vortex. Those latter particles are extracted from the cyclone separator via the lower outlet port. As a general statement, the coarser particles or solids experience greater centrifugal settling forces than the finer solids. Hence, known cyclone separators separate particles on the basis of particle size.
Some prior art cyclone separators utilize vortex finders which move axially within the cylindrical and/or frustoconical regions. By axially positioning the vortex finder, one can regulate the amount of overflow or fluid extracted from the cyclone separator via the vortex finder and upper outlet port. In a similar fashion, some cyclones utilize valves which restrict flow through the lower outlet port and hence change the balance of forces within the cyclone body to affect the type of particles entrained within the forced vortex and exiting the upper outlet port via the vortex finder. U.S. Pat. No. 3,259,246 by Stavenger and U.S. Pat. No. 4,414,112 by Simpson et al. disclose such cyclone separators.
Some cyclone separators include a secondary outlet port, known in the art as a "sidearm," to extract a specified flow of slurry from the internal regions of the cyclone separator at an axially intermediate location. U.S. Pat. No. 2,981,413 by Fitch extracts a flow of slurry from such a sidearm port and reinjects the same into the cyclone and providing additional motive power for the slurry swirling in the cylindrical region of the cyclone separator. U.S. Pat. Nos. 2,418,061 by Weinberger; U.S. Pat. No. 3,533,506 by Carr; and U.S. Pat. No 4,097,375 by Molitor disclose side ports extracting a flow of slurry from the frustoconical region However, each sidearm port in those known cyclone separators is covered by a screen or a permeable or porous media. The screen allows particles of only a certain size to pass therethrough. The porous media allows only fluid to pass therethrough. Also, those sidearm ports are located in the frustoconical region.
In general, the ability of the cyclone separator to segregate particles depends upon the particle size. However, a publication entitled "Theory of Hydrocyclone Operation and its Modifications for Application to the Concentration of Underwater Heavy Mineral Sand Deposits," by A. Hayatdavoudi, published May 15, 1975, discloses that the separation of sand and magnetite, being fluidly suspended in water and having approximately the same particle size but different densities, can be accomplished in a glass bodied, hydrocyclone due to the development of two separate descending spirals, one sand and the other magnetite. Further, it was noted that an adjustable scrapper tube inserted into a sidearm located tangentially to the wall of the hydrocyclone may result in withdrawal of a major portion of the sand and hence the successful concentration and recovery of the magnetite. However, the publication states that the precise location of the head of the scrapper tube depends on a number of mathematical models and on the fundamental operation of the hydrocyclone. Also, it is specifically recognized in that disclosure that the interaction of the many variables of operation within the hydrocyclone does not provide a clear picture of the complicated phenomenon occurring therein.